In the production of phosphoric acid by the wet process, large amounts of calcium sulfate are formed. Depending on the particular process and the reaction conditions, it is formed as the dihydrate, the hemihydrate or anhydrite. In most processes these precipitates are separated and discarded as waste. Hitherto, it was only after complicated purification processes that these precipitates could be used in the construction industry for purposes such as gypsum plaster.
Thus, the purification of phosphogypsum by the so-called hydrothermal process is known, in which the purification is carried out by conversion into calcium sulfate hemihydrate. The calcium sulfate so prepared can be compressed into gypsum bricks as in German Patent No. 23 17 049 by mixing it with a quantity of water which at least approaches, and at most is equal to, the stoichiometric amount required for hydration to the calcium sulfate dihydrate, and compression molding at a temperature between 45.degree. and 90.degree. C.
Another process for preparation of building materials from gypsum is described in German No. OS 28 05 523 in which 30-100% of calcined gypsum (plaster) having a Blaine surface of 500 to 15,000 cm.sup.2 /g is added to the moist dihydrate. Before shaping by molding can be successful, the phosphogypsum has to be purified by a complicated procedure (cf. the cited German OS, pp. 8-9 (3-4)), the plaster portion has to be prepared from the phosphogypsum by further process steps involving drying and calcining (loc. cit. pp. 11-12 (6-7)), and the wet dihydrate and plaster must be mixed in a ratio which is calculated by formula corresponding to the water content which is chosen to be between 0 and 15%.
The process of the invention for preparing shaped articles of gypsum differs from those mentioned above in that neither purification nor calcination of the phosphogypsum is necessary. Accordingly, there is no wastewater produced and no thermal energy is required.
The process of the invention is preferably used for calcium sulfate hemihydrate which is produced in the phosphoric acid processes of German OS Nos. 22 33 189, OS 23 51 031, and/or OS 24 12 503.
In these processes, it is true, a direct compression molding of the recovered hemihydrate has been possible, but the shaped articles produced have hitherto not attained the hardness which is required, for example, for the erection of bearing walls (cf. Exaxmple 2, page 8).